1. Field of the Invention
The present invention relates generally to the field of optical superheterodyne receivers and, more particularly, the present invention relates to a method for setting the local oscillator of an optical superheterodyne receiver.
2. Description of the Related Art
In optical superheterodyne receivers, the strong, unmodulated light of the local oscillator is added in the receiver to the modulated light of the data signal generated in a transmission laser. This demodulation scheme provides higher sensitivity and selectivity than direct reception for selection of a specific transmission channel.
In these systems, binary Frequency Shift Keying (FSK) has proven to be an especially promising method of modulation because semiconductor transmission lasers can be frequency modulated simply by varying the current. Optimum heterodyning for this modulation requires a polarization matching of the local oscillator signal with the data signal. Data-Synchronous Polarization Keying (DSP) is of interest for this purpose in addition to polarization control and polarization diversity. (see, i.e. R. Noe et al., Electronics Letters 25 (1989) 1, 4-5 and .sctn..sctn.IV and V by R. Noe Journal Of Lightwave Technology 9 (1991) 10. This is especially true of distributor systems having a relatively small number of transmitters and a relatively large number of subscriber receivers. DSP can be accomplished by either data-induced polarization keying with a passive birefringent component or actively by keying with a polarization modulator.
In DSP, the two characteristic frequencies are received with polarizations that are or orthogonal relative to one another. When keying is implemented at, for example, 50 transmitters and the polarization is solved for each of, for example, 10,000 receivers, the cost per subscriber becomes minimal.
Optical superheterodyne receivers require Automatic Frequency Control (AFC) of the Intermediate frequency in order to assure optimum performance. Automatic Frequency Controls have a capture range within which the rated value of the Intermediate Frequency, i.e. the correct intermediate frequency is realized after the AFC is switched on. Because the frequency uncertainty of the transmitter, or the local oscillator is generally greater than the capture range, presetting of the local oscillator frequency is required before switching the AFC. The local oscillator frequency is normally varied in a ramp form as a function of time for this purpose. The presence of the Intermediate Frequency signal and its position relative to the local oscillator frequency is determined with a level detector or frequency discriminator and the local oscillator frequency is set such that the signal is located within the capture range. The AFC is then switched on.
Various methods for automatic intermediate frequency control of DSP signals already exist. However, the only known methods for non-DSP signals require presetting the local oscillator within the capture range of the AFC (see S. Yamazaki et al., "Tunable Optical Heterodyne Receiver For Coherent FDM Broadcasting Systems", Proc. European Co. and F. on Optical Communications (ECOC 1988), Brighton, 1988, IEEE Conference Publication No. 292, Part I, Pages 86-89).